Method of producing vacuum tube electrodes



L. GOODALE L METHOD OF PRODUCING VACUUM TUBE ELECTRODES Filed March 11,1942 INVENTOR Zrxwv 000 445 ATTORNEY Oct. 29, 1946.

Patented Oct. 29, 1946 METHOD OF PRODUCING VACUUM TUBE" ELECTRODES LynnC. Goodale, Newark, N. J., assignor to Federal Telephone and RadioCorporation, a,

corporation of Delaware Application March 11, 1942, Serial No.- 434,269

9 Claims.

This invention'relates to vacuum tube filaments and more particularly tothe method of making spiral vacuum tube filaments and the filaments somade.

In vacuum tube filaments, particularly filaentary cathodes for tubesdesigned to operate at high frequencies, the spacing requirements of thefilament may be very critical. For example, in a certain type of tubethe spacing between the filament and grid is in the order of 0.040 inch.In'such tubes the filament is generally of relatively fine wire and maywarp or sag during carburizing or setting thereof. Such warping orsagging may upset the entire characteristics of the tube. 1

:It is Fanobject of my invention to overcome the above diificulties byuse of a new method of producing spiral filaments.

Furthermorapractice of mynew methodmay produce a filament which isca'rburized only on,

the outside and a further object of my invention is the production, ofspiral filamentscarburized only on the outer surface of the spiral.

According to a feature of my invention, spiral filaments, single ormultispiral, areformed and are then screwed into a sleeve made of:carbon or of other material if setting alone is desired,

threaded to receive the filament: The whole;

assembly is then heated .to a high, temperature. This heating serves toset the filament land if a carbon sleeve is used alsov carburizes for,partially carburizes the filament. Preferably, for carburizing, thespiral filament is lubricated with colloidal graphite while screwinginto the sleeve..

Since the spiral filament is held firmly in place by the grooved sleeve,the filament has no opportunity to sag or warp during the heatinglthermore, since only the outer surface of the;

filament spiral is in contact with the block or sleeve, this outersurfaceis carburized to a greater extent than the inner surface when acarbon block is used.

A better understanding, of my invention and the objects and featuresthereofmay be had from the particular description thereof made withreference to the accompanying drawing, in which Fig. 1 illustrates anarrangement for practicing the process andproducing filamentsinaccordancewith my invention; H V

Fig. 2 is an illustration of a single spiral filament;

Figs, 3 and 4 are elevations and end views respectively of quadra-filarfilament, and

Fig. 5 is a cross-sectional View of a filament wire indicating the outersurface.

earburization effect on the tungsten and often they are thoriated forthe, purpose ofincreasing their electron emission.

The wires may be in ductile form as originally wound, and' it is thendesirable to have them set before incorporating them in the Vacuum tubeto preventdistortion or warping of the wires due to strains that mayexist therein. This socalled 'setting of the wires-is accomplished byheating the wires to a temperature near the melting point-of the wiresor at least to incande'scense. After such a heat treatment, tung-v stenwires in particular, are found to be more rigid than previously althoughthey are also more brittle. Furthermore, in the case of thoriatedtungsten filaments it is often desired to treat these filaments toproduce carburization. This treatment also calls for heating at arelatively high temperature after the wire has been covered with a,carbon composition. During such heat treatment the filament may sag ordeform, thus producing errors in the spacing of the filament withrespect to other electrodes in the vacuum tubes. Since, in many tubesthese spacings between electrodes are relatively small and verycritical, this variation in spacing tends to produce errors in thecharacteristics of the tubes. In accordance with my invention in orderto prevent these faults in filaments, the spiral filament consisting ofcoils l0 and l I, is screwed into a previously prepared block or sleeveI 3, which is provided with an internal threaded groove I 4 of theproper, dimension and pitch to receive the spiral and hold it in placeduring heating. The entire unit is then inserted into a heater I5, whichmay be for example, a coil supplied with heating energy over leads I6,l1. During the heating process the spiral grooves I4 hold the wiresfirmlyin place so that after the wire has been set it, will be shapedproperly and not deformed.

If sleeve [3 is made of carbon then the filament Ill, II is carburizedat the same time that the setting takes place. When carburization isdesired the wire filaments are preferably lubriin accordance with myinvention will generally carburized on a section corresponding to theouter edge of the spiral coil. Of course, a certain degree ofcarburization may take place on the inner surface of the coil as well,due to the graphite lubrication. However, the greatest heat is on theoutside as well as the greatest quantity of carbon and for this reasonthe carburization will be largely on the outersurface of the spiralfilament.

It is clear that the blocks or sleeves used for carburizing the filamentmay be reused for treatment of a plurality of individual filaments andneed not be replaced newior each filament to be treated.

It should be noted that in many cases the wire filaments might tend toexpand upon attempt to screw them into thesleeve in the manner of cer-vtainforms-of spring clutches. In order to avoid this difiicultyshouldthe wires be sufficiently flexible to tend to expand in this manner, itis advis-. able that the threading action be taken at the leading edgeof the spiral so that instead of producing a tendency to expand thetendency will be to contract the filament as it is screwedinto place. Inmany cases the supporting rods [2 will have sufiicient rigidity to servethis purpose. However, in. event the supporting rods are notsufiiciently rigid, for this purpose, a special tool may be providedwhich will befastened to the filament at the leading edge for insertingit into the sleeve.

Fig. Zillustrates asingle spiral filament. It is clear that thefeaturesof my invention. maybe. applied not only tobi-filar filaments ofthedouble spiral, as shown in Fig. 1, but to any spiral filamentsregardless otthe. number of spirals therein. In. this figurethesinglespiral is shown at 29,. provided with a supporting rod and returnlead.21. for the filament.

In Figs. 3 and 4 is shown what: is called a quadraefilar filament. Inthis arrangement there is provided four separate spirals 3i, 32, 33, and34; Spirals 3.1, 3-2, together with a supportingrod 35: form onebi-filarfilament winding similar to that shownin Fig. 1. Filaments. 33 and .34,to-

gether with supporting rods 36., form. a second bi-filar filamentsimilar. to that shown in.Fig. 1. The two filament elements are. thenthreaded together to form the completebi+filar unit as shown in Figs. 3and l. Inpractice, the four. separate filament leads of-3I, 32, 33 and34; and the separate supporting rod leads areoften brought out of thetube socket to form fixed terminals. Usually then, the two doublespirals whichareprovided are'then connected in series, for example,

by connecting together wires 32 and-33 inthe socket, and .the energyissuppliedacross leads 3| and 34. The quadra-filar filament then formsafour-threaded spiralwhich may be screwed'intoaccompanies the normalheating" andsetting of the filaments, but they produce improvedemissionwhich in some cases is equivalent to almost twice that of the emissionfrom a filament formed and carburized in the ordinary way.

It should, of course, be understood that the above description merelydescribed a preferred manner inmaking and using my invention. Also, itshould be understood that in most cases the filament wires will not bemade of as heavy material as shown in the drawing and will generally beof a, smaller size.

The carburization may be achieved by supporting the filament on acentral mandrel of carbon or some other material such as ceramics whichwill withstand the necessary heat. The unit may then be painted withcolloidal graphite or imbedded in granulated carbon and heated tocarburizing temperature. Threads or other depressions in the mandrel maybe provided and the filament thus held in position during thesimultaneous heating and carburization.

It should, also, be distinctly understood that my invention includes notonly treatment of: spiral filaments in this manner, butthe simultaneouscarburization and setting of filaments by insertion in any properlyformed. carbon block and subsequent heat treatment or by other treat-.

ments as explained above regardless of the fila-v ment form.

. While the specific description has beendirectedto filamentarycathodes, it, should be understoodv that the invention applies to. anyform of filamentary electrodes, for example to filamentary gridelectrodes.

Many other modifications and examples ofi my. invention will be apparentto thoseskilled in .the

electrodes comprising. forming a spiral of fila-. ment wire, screwingsaid spiral into a car bonaceoussleeve provided with a spiral groove toreceive saidfilament spiral, and heating the assembledfilament andsleeve to a temperature 1 sufiiciently high toset said spiral andthusremove internal stresses which might cause deformationof saidfilament, and to cause carburization of at least that portion of thefilament which is incontact with the sleeve.

2. The method of simultaneously carburizing and setting a vacuum tubefilamentary electrode Whichcomprises forming the desired shape offilament from ductile filament wire, placing the shaped filament wire ina carbon retaining means a previously formed to retain said filament insaid desired shape, and heating said assembled filament and retainingmeans to carburizing temperature.

' 3. Th method of'producing carburized' spiral filaments comprisingforming a spiral of the desired shape of thoriated filament wire,screwing said spiral 'into a previously prepared carbon sleeve providedwith a-spiral groove tofit said filament spiral, and heating theassembled-film merit and sleeve to produce at least apartial- 7. Themethod according to claim 3, wherein said spiral is formed in the shapeof a quadrafilar filament winding.

8. The method of carburizing filamentary vacuum tube electrodes, whichcomprises forming the filamentary electrode in the shape of a spiral,supporting the electrode by engagement of the outer surface of saidelectrode with interior portions of a hollow member formed of carbon andsubjecting the whole assembly to a, carburizing 10 heat, whereby theouter surface of said electrode is carburized.

9. In the manufacture of a. vacuum tube electrode, a, method forproducing limited superficial carburization of a preselected discretesurface area of said electrode that comprises substantially rigidlysupporting the electrode by engagement with interior parts of a tubularmember formed of a carbonaceous substance in a manner such that thesurface area to be carburized is in contact with the carbonaceoussubstance, and heating the electrode while so supported at a temperaturesufficient to effect carburization of said area.

LYNN C. GOODALE.

